In the power generation industry, industrial turbine systems are used to generate electricity. The turbine systems include, among other things, an industrial turbine (e.g., a gas turbine), a supply tank, a supply skid, a metering skid, a pump, relief valves, shut-off valves, and connecting piping. The supply skid and metering skid cooperate to supply or feed the industrial turbine with a liquid fuel such as, for example, diesel fuel, jet fuel, kerosene, a gaseous fuel, and the like.
The characteristics of the fuel are such that a sudden closure of a shut-off valve (i.e., a stop valve) in the turbine system results in a rapid rise in pressure within the system often resulting in a pressure spike. The rise in pressure, or pressure spike, often continues until a pump (e.g., a positive displacement or centrifugal pump) driving the fuel through the system can be effectively shut down or until a relief valve opens. In addition to the continued operation of the pump, the pressure can also rise as a result of the inertia of the pump, control sensing delays, the “water hammer” effect, and the incompressible nature of the fuel being used.
The rate at which the pressure rises in the turbine system is often compounded when the shut-off valve closes very quickly. For example, in some cases, a shut-off valve used in the turbine system has to be very fast in order to protect the turbine from “overspeed” in certain operational and fault scenarios. The total shut-off time for the shut-off valve can be mere milliseconds. While closing the shut-off valve this quickly meets the requirements for discontinuing fuel flow to the turbine under emergency conditions, a rapid pressure rise of more than one hundred pounds per square inch (psi) per millisecond can be generated just upstream of the shut-off valve. With the pressure rising so quickly, some form of pressure relief must be provided before the pressure limitations of the equipment (e.g., pipes, fittings, etc.) are exceeded.
In addition to the rapidly rising pressure dilemma, the abrupt closure of the shut-off valve also triggers a large “inertial” pressure oscillation in the piping, which can be seventy feet in length or more, between the supply skid and the metering skid in some installations. The pressure oscillation can potentially damage sensitive equipment used on the skid (e.g., flow and pressure sensors, a filter canister, etc.).
While some relief valves have been designed to open very quickly to relieve pressure, these valves typically only provide a limited amount of flow. The limited amount of flow make these valves unusable in many turbine applications. As those skilled in the art can appreciate, increasing the flow area of a relief valve and reducing the opening time are conflicting design parameters. For example, attempts were made to design a relief valve that opens very quickly (i.e., in milliseconds or microseconds) to inhibit or prevent pressure spikes and inertial pressure oscillations while maintaining a sufficient effective maximum flow area to accommodate the amount of fuel (e.g., 150 gallons per minute) supplied by the fuel pump. While one embodiment of the relief valve was large enough to provide a full flow capability, the valve was limited to an opening response time of approximately forty milliseconds, which was simply too slow. Further optimization of the relief valve to improve on the forty millisecond response would likely create the potential for undesirable system instability due to relief valve chatter.
Other possible solutions have been tried by industry. In one instance, to address the transient pressure spike problem, two five gallon gas-charged bladder accumulators were installed upstream of the shut-off valves. These types of accumulators not only provide very fast pressure relief, they also have very large flow absorption rates. However, the pressure in the gas-charged bladder accumulators changes as the temperature varies and this can cause problems. For example, to mitigate the effects of changing gas charge pressure due to temperature, a temperature control system had to be added to regulate the temperature of the accumulators. To ensure that the gas charge remains within allowable limits, periodic monitoring and maintenance of the gas pressure is required. This solution proved costly, complex, potentially unreliable, and resulted in a large, costly peripheral system. As a result, the addition of multiple accumulators to the turbine system to achieve higher reliability would not be fruitful in most applications. In fact, the use of accumulators increases the potential for gas leakage and lowers the overall reliability of the system.
In another attempt to deal with pressure spikes or transient pressures, three-way shut-off valves have been used in turbine systems. Unfortunately, these devices are generally not very fast (e.g., they need 100 milliseconds to accomplish shut-off) and are not easily scalable to larger sizes while maintaining desired cost and package size. Also, these designs are generally limited to liquid fuel or water applications. Thus, an apparatus that can eliminate or mitigate the effects of pressure spikes and transient pressures in a cost effective, reliable, and efficient manner would be desirable. The invention provides such an apparatus. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.